Several years ago the Voyager spacecraft neared the edge of the Solar System, where the solar wind and magnetic field started to be influenced by the pressure from the interstellar medium that surrounds them. But the expected breakthrough to interstellar space appeared to be indefinitely put on hold; instead, the particles and magnetic field lines in the area seemed to be sending mixed signals about the Voyagers' escape. At today's meeting of the American Geophysical Union, scientists offered an explanation: the durable spacecraft ran into a region that nobody predicted.

The Voyager probes were sent on a grand tour of the outer planets over 35 years ago. After a series of staggeringly successful visits to the planets, the probes shot out beyond the most distant of them toward the edges of the Solar System. Scientists expected that as they neared the edge, we'd see the charge particles of the solar wind changing direction as the interstellar medium alters the direction of the Sun's magnetic field. But while some aspects of the Voyager's environment have changed, we've not seen any clear indication that it has left the Solar System. The solar wind actually seems to be grinding to a halt.

Today's announcement clarifies that the confusion was caused by the fact that nature didn't think much of physicists' expectations. Instead, there's an additional region near our Solar System's boundary that hadn't been predicted.

Within the Solar System, the environment is dominated by the solar magnetic field and a flow of charged particles sent out by the Sun (called the solar wind). Interstellar space has its own flow of particles in the form of low-energy cosmic rays, which the Sun's magnetic field deflects away from us. There's also an interstellar magnetic field with field lines oriented in different directions to our Sun's.

Researchers expected the Voyagers would reach a relatively clear boundary between the Solar System and interstellar space. The Sun's magnetic field would first shift directions, then be left behind and the interstellar one would be detected. At the same time, we'd see the loss of the solar wind and start seeing the first low-energy cosmic rays.

As expected, a few years back, the Voyagers reached a region where the interstellar medium forced the Sun's magnetic field lines to curve north. But the solar wind refused to follow suit. Instead of flowing north, the solar wind slowed to a halt while the cosmic rays were missing in action.

Enlarge/ Particles originating from the Sun (purple) dropped even as those from interstellar space (blue) rose.

Over the summer, as Voyager 1 approached 122 astronomical units from the Sun, that started to change. Arik Posner of the Voyager team said that, starting in late July, Voyager 1 detected a sudden drop in the presence of particles from the solar wind, which went down by half. At the same time, the first low-energy cosmic rays filtered in. A few days later things returned to normal. A second drop occurred on August 15 and then, on August 28, things underwent a permanent shift. According to Tom Krimigis, particles originating from the Sun dropped by about 1,000-fold. Low-energy cosmic rays rose and stayed elevated.

That's a bit more expected; what wasn't expected is that the magnetic field of the Sun hasn't changed at the same time. Since it's been in the heliosheath, Voyager 1 has been seeing the magnetic field lines that originate from the southern hemisphere of the Sun. Those have been twisted around to face north by the pressure from the interstellar magnetic field. The orientation of these field lines hasn't changed at all, even as the particle counts shifted rapidly. In fact, the magnetic field lines appeared to get stronger and the increase in strength occurred in concert with the changes in particle counts.

Enlarge/ Even as the flow of solar particles dropped, the Sun's magnetic field strength appeared to rise.

The researchers suspect they've reached a region of the solar-interstellar boundary that nobody had predicted. In this area, the magnetic field lines of the Sun link up with those of the interstellar field. Scientists are calling this linkage a "highway" for particles to travel along. It lets solar wind particles escape more readily, causing the drop in their intensity. And it opens the door for low-energy cosmic rays to slip in to our Solar System, which is why Voyager 1 is seeing so many of them.

According the researchers at the press conference that announced these results, most steady-state models of the Solar System failed to predict anything like this. A few models did have a feature like this, but it was only a transient one that appeared at certain times of the solar cycle.

Voyager 2 is trailing behind and hasn't reached this region yet. Researchers suspect it may take several years for Voyager 1 to clear this area entirely, after which it will finally reach interstellar space.

Promoted Comments

This is probably (for me at least) one of the more fascinating things NASA has ever done. To be alive in a time when we are able to reach the edge our own solar system and beyond, and receive data on that event is so awesome. Personally, I never expected the Voyagers to make it this far when they first launched. I figured they'd malfunction, get struck by some space debris, etc. But wow, they are nearly beyond our own solar system. Amazing technical feat NASA.

Researchers suspect it may take several years for Voyager 1 to clear this area entirely, after which it will finally reach interstellar space.

Well, by some definitions, anyway. If you define the edge of the Solar System as the heliopause, then yes. But if you define the edge of the solar system as the point at which the Sun can no longer hold worlds in orbit around it, then you're talking about the Oort cloud. In which case Voyager won't reach interstellar space for a very long time. Like 14,000 years.

I wonder if Curiosity will still be trundling around Mars in 30 years?

The 110 watt RTG is 1/4 as powerful as what is on the Voyager craft - but the power required for radio transmission is lower I think. It also has batteries that are charged for peak times.

At 30 years it most likely will not be roving around anymore, power demands would be too severe for the degraded systems. If parked in a really good spot there could be useful stationary missions for it, don't know if that could stretch that long though.

"Today's announcement makes clear the confusion was caused by the fact that nature didn't think much of physicists' expectations."

Shouldn't we be use to his by now. It should be at the top of any checklist by default at this point. Stop thinking you know everything science man!!! (shakes fist)

I'd hardly think anyone on this team thinks we/they know everything. If we knew what we would find, why launch a probe? The act of doing the experiment is in itself a statement that we don't know and want to learn.

I wonder if Curiosity will still be trundling around Mars in 30 years?

The 110 watt RTG is 1/4 as powerful as what is on the Voyager craft - but the power required for radio transmission is lower I think. It also has batteries that are charged for peak times.

At 30 years it most likely will not be roving around anymore, power demands would be too severe for the degraded systems. If parked in a really good spot there could be useful stationary missions for it, don't know if that could stretch that long though.

How long will the nuclear fuel last?

I wonder if they could reprogram it to be a radio relay for future missions if the mechanics give out?

So, could it be that since we don't have any close by stellar neighbors (Proxima Centauri being, what, 4 light years away?) it allows the Sun to have more of a magnetic field than we see around other stars? Almost like a gas expands to fill the container in, the magnetic field expands until it has other things counteract it? Still, really interesting that we're still getting data back from them. Definitely says a lot about NASA back in it's heyday. I wonder if it still has the ability to take pictures, and if it sent any back if they'd be worth seeing.

Researchers suspect it may take several years for Voyager 1 to clear this area entirely, after which it will finally reach interstellar space.

Well, by some definitions, anyway. If you define the edge of the Solar System as the heliopause, then yes. But if you define the edge of the solar system as the point at which the Sun can no longer hold worlds in orbit around it, then you're talking about the Oort cloud. In which case Voyager won't reach interstellar space for a very long time. Like 14,000 years.

I think the thing that's amazing is objects light years away can still have a significant influence.

I wonder if Curiosity will still be trundling around Mars in 30 years?

The 110 watt RTG is 1/4 as powerful as what is on the Voyager craft - but the power required for radio transmission is lower I think. It also has batteries that are charged for peak times.

At 30 years it most likely will not be roving around anymore, power demands would be too severe for the degraded systems. If parked in a really good spot there could be useful stationary missions for it, don't know if that could stretch that long though.

How long will the nuclear fuel last?

I wonder if they could reprogram it to be a radio relay for future missions if the mechanics give out?

Probably not. Curiosity relays most of its data through the orbiters instead of sending it directly because they're both higher power (giant solar arrays are much more feasible in zero g) and consequently have a much higher bit rate and have line of sight to the Earth much more frequently.

I wonder if Curiosity will still be trundling around Mars in 30 years?

The 110 watt RTG is 1/4 as powerful as what is on the Voyager craft - but the power required for radio transmission is lower I think. It also has batteries that are charged for peak times.

At 30 years it most likely will not be roving around anymore, power demands would be too severe for the degraded systems. If parked in a really good spot there could be useful stationary missions for it, don't know if that could stretch that long though.

How long will the nuclear fuel last?

I wonder if they could reprogram it to be a radio relay for future missions if the mechanics give out?

It'd only be useful for short range, basically line of sight. We'd have to visit the area again. And at that point the satellite relays would still be better choices.

If they could re-purpose some instruments they might be able to create a makeshift seismic monitoring station, if they can measure the amount that some of the detailed instruments move. That could be helpful. NASA can really find ways around problems that arise. Look at how they've managed to salvage Hubble.

This is probably (for me at least) one of the more fascinating things NASA has ever done. To be alive in a time when we are able to reach the edge our own solar system and beyond, and receive data on that event is so awesome. Personally, I never expected the Voyagers to make it this far when they first launched. I figured they'd malfunction, get struck by some space debris, etc. But wow, they are nearly beyond our own solar system. Amazing technical feat NASA.

Could this transitional area that V'ger 1 has entered be similar to our magnetosphere above Earth? Could there possibly be interstellar auroras caused by the cosmic radiation? Really is a shame that the V'gers don't have enough power to take pictures still.

Great article - I definitely plan on following any and all activity from these spacecraft as long as they keep sending info back!

Original Article wrote:

A second drop occurred on August 15 and then, on August 28, things underwent a permanent shift. According to Tom Krimigis, articles originating from the Sun dropped by about 1,000-fold. Low-energy cosmic rays rose and stayed elevated.

This is probably (for me at least) one of the more fascinating things NASA has ever done. To be alive in a time when we are able to reach the edge our own solar system and beyond, and receive data on that event is so awesome. Personally, I never expected the Voyagers to make it this far when they first launched. I figured they'd malfunction, get struck by some space debris, etc. But wow, they are nearly beyond our own solar system. Amazing technical feat NASA.

This.

Honestly, the discovery of the Higgs was "oh, nice, good," but also sort of boring because it was what we were searching for. This, though? Finding out something completely unexpected and contrary to our models? This is the exciting stuff!

Am I the only one, who whenever I hear about new surprising discoveries from the voyagers, get a mental picture of the probes hitting the game-world invisible wall and are now bouncing back? Just to imagine the expression on the faces of my friends in CERN and NASA when they try to explain that to me.

I wonder if Curiosity will still be trundling around Mars in 30 years?

I believe I read its nuclear fuel (for keeping electrics warm enough) would be extremely lucky to make it to 5 years.

5 years of roving. And use of the rover would have no impact on anything other than the batteries. The power source itself could last a lot longer, but the usefulness of a rover and can't rove is debatable - communication with it isn't free.

Am I the only one, who whenever I hear about new surprising discoveries from the voyagers, get a mental picture of the probes hitting the game-world invisible wall and are now bouncing back? Just to imagine the expression on the faces of my friends in CERN and NASA when they try to explain that to me.

Once the bounce occurs it triggers a game reset. Probably around December 21st or so....

I forget, have we put up any other probes that will end up heading out of the system with more modern instruments? I ask, because if we don't, or don't have any planned, it's a shame. We could probably get some really good data in a few decades if we launch out some new probes, or a couple of small probes in a formation close enough for good interferometery, and send them out on a high energy trajectory. Plus, I think we need to send a few up out of the plane of the solar system, since that region is undoubtedly somewhat different.